The relative effectiveness of several alkyl lead compounds, inorganic lead, methyl mercury, and inorganic mercury as inhibitors of caudate nucleus adenylate cyclase activities has been investigated. Methyl mercury, inorganic mercury, and several alkyl lead compounds are relatively effective inhibitors of the dopamine-sensitive adenylate cyclase, whereas inorganic lead was not an effective inhibitor. Organic lead salts are of particular interest as inhibitors of the dopamine-sensitive adenylate cyclase, because those salts with hydrophobic substituents are more effective inhibitors than those with greater water solubility. For instance, tri-n-butyl lead acetate is more inhibitory than tri-n-propyl lead acetate, which is more inhibitory than triethyl lead acetate. From another point of view, the extent of aryl (phenyl) substitution of lead correlates in a positive fashion with the inhibitory effectiveness of the alkyl lead salt. The dopamine sensitive form of adenylate cyclase is usually inhibited at alkyl lead concentrations lower than those required for inhibition of basal adenylate cyclase activity.